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Cite this: Chem. Commun., 2015,
51, 10819
Organoaqueous calcium chloride electrolytes for
capacitive charge storage in carbon nanotubes at
sub-zero-temperatures†
Received 13th April 2015,
Accepted 26th May 2015
Yun Gao,ab Zhanbin Qin,b Li Guan,a Xiaomian Wangb and George Z. Chen*ac
DOI: 10.1039/c5cc03048j
www.rsc.org/chemcomm
Solutions of calcium chloride in mixed water and formamide are
excellent electrolytes for capacitive charge storage in partially oxidised
carbon nanotubes at unprecedented sub-zero-temperatures (e.g. 67%
capacitance retention at
60 8C).
Electrochemical capacitors (ECs), also known as supercapacitors,
are fast emerging energy storage devices.1 Current EC research is
mainly focused on new electrode materials,2 but electrolytes are
also an essential factor deserving more attention. Electrolytes in
ECs can be broadly classified into non-aqueous and aqueous.
Non-aqueous electrolytes3 are based on either organic solvents,
such as propylene carbonate, acetonitrile, or ionic liquids (ILs).
Organic electrolytes can offer wide potential windows that are
needed for high energy storage capacity, but also have high
volatility and low flaming points, causing safety concerns. ILs are
much less volatile but they are relatively expensive and also too
viscous to offer good conductivity. Aqueous electrolytes4 are
usually concentrated solutions of salts, acids or bases. They are
restricted by narrow potential windows due to decomposition of
water, but have many advantages over non-aqueous electrolytes,
such as high conductivity, affordable cost, and low toxicity.
These merits of water based electrolytes make them particularly
attractive for large scale and stationary applications such as
storage of electricity generated from wind turbine farms.
Nevertheless, it is still a challenge to use aqueous electrolytes.
Water freezes at 0 1C, while winds are often stronger and more
frequent in places where cold winter often experiences sub-zero
temperatures. Hence, for low temperature applications, the
a
Department of Chemical and Environmental Engineering, and Energy and
Sustainability Research Division, Faculty of Engineering, The University of
Nottingham, Nottingham NG7 2RD, UK. E-mail: [email protected]
b
Department of Chemical Engineering, North China University of Science and
Technology, Tangshan, Hebei 063009, P. R. China
c
Department of Chemical and Environmental Engineering, and Centre for
Sustainable Energy Technologies, Faculty of Science and Engineering, The
University of Nottingham Ningbo China, Ningbo 315100, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cc03048j
This journal is © The Royal Society of Chemistry 2015
electrolytes used in supercapacitors (and batteries) are commonly
non-aqueous,5 whilst aqueous electrolytes are rarely considered.6
However, the very recent finding of a specific capacitance value of
30 F g 1 is encouraging by testing the MnO2–carbon composites in
mixed water, ethylene glycol and (NH4)2SO4 at 30 1C.6a
It is known that mixing two liquids may lead to eutectic
behaviour if hydrogen bonds are involved.7 In this work, a simple
mixture of formamide (FA) and water (FA–H2O, 1 : 1, v : v) was found
to start and finish freezing between 32 1C (liquidus temperature)
and 44 1C (solidus temperature). This liquidus–solidus temperature range (LSTR) is much lower than the melting point (m.p.) of FA
(2 1C) or H2O. This observation can be explained as follows. First, the
vapor pressure of any component in a mixture is lower than that of
the component alone from the viewpoint of physical evaporation,
leading to a LSTR lower than the m.p. of each component. More
importantly, FA and H2O are both hydrogen bond donors and
acceptors. As a result, hydrogen bonds are plentiful in the FA–H2O
mixture and can contribute to lowering the LSTR. (Note: we cannot
find the FA–H2O binary phase diagram in the literature and therefore constructed it in this work as presented in Fig. S1 in the ESI.†)
The low LSTR of the FA–H2O mixture promoted us to use it
to dissolve CaCl2 for the preparation of low-temperature electrolytes.
CaCl2 was selected for several reasons. Although not used in past
supercapacitor research, CaCl2 is a low cost salt with its toxicity
(as animal food additive) and water solubility comparable to those of
NaCl. Also, for road de-icing in winter, CaCl2 is widely used mainly
because it gives a eutectic temperature of 51.6 1C, whilst NaCl
could only reach 21.1 1C.6c This ability of CaCl2 results possibly
from the high affinity between calcium and oxygen. In thermodynamic terms, CaO is the most stable metal oxide next to CeO2.
This high calcium–oxygen affinity can be translated to that
between Ca2+ ions and the oxygen atoms in water molecules.
For the same reason, CaCl2 is extremely hygroscopic and often
used as a drying agent in laboratory.
We believe that this unique nature of CaCl2 can make it a better
electrolyte than other common salts, such as KCl and Na2SO4,
for capacitive charge storage on carbon surfaces enriched with
e.g. carbonyl, carboxyl and hydroxyl groups or simply oxy-groups.
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In line with this thought, carbon nanotubes (CNTs) were selected for
electrochemical tests because CNTs are well-known, particularly after
partial oxidation in acids, to possess surface oxy-groups.8 The affinity
of Ca2+ ions to oxy-groups explains the unique Ca2+ selectivity of the
ionophore A23187 over alkali metal ions (see ESI†). Furthermore, a
pure carbon surface is hydrophobic, but because of the oxy-groups,
the CNT surfaces are amphiphilic in nature,8c and may be better
accessed by an organoaqueous electrolyte.
Three CaCl2 solutions were prepared using H2O, FA, or FA–H2O
(vol ratio, 1 : 1 which was selected to compromise CaCl2 solubility,
viscosity and liquid temperature range). As expected, CaCl2 addition
lowered the liquidus temperature to 16 1C for CaCl2–H2O
(2.0 mol L 1), 41 1C for CaCl2–FA (1.0 mol L 1), and below
60 1C for CaCl2–FA–H2O, (2.0 mol L 1), respectively.
To prepare the CNT working electrode, 20 mL aqueous
suspension of acid treated CNTs8 with PTFE (binder, 5 wt%
after drying) was dropped on a 5 mm diameter graphite disc
electrode, giving rise to a CNT loading of ca. 0.2 mg cm 2. After
drying in air (45 h), the CNT electrode was used to record
cyclic voltammograms (CVs) in different electrolytes. A graphite
rod and the Ag/AgCl couple were used as the counter and
reference electrodes, respectively (see ESI† for more details of
the three electrode cell).
In this work, temperature control was achieved in two ways.
To determine the liquidus and solidus temperatures, an ultralow
temperature thermostat bath was used. For cyclic voltammetry,
the cell containing 10 mL of the electrolyte was placed in a
thermo-container filled with dry ice ( 78.5 1C). A thermocouple
was inserted in the electrolyte to monitor the temperature. It was
observed that the electrolyte temperature dropped fairly slowly,
e.g. o1 1C min 1 below 55 1C, to enable several CV cycles at a
designated temperature and a sufficiently high potential scan
rate, e.g. 100 mV s 1 (20 s per cycle).
Fig. 1 compares the CVs and specific capacitances of the
CNT electrode measured in different electrolytes at 20 1C and
60 1C. Fig. 1a shows that at 20 1C the CNT electrode behaved
almost identically in the CaCl2–H2O and CaCl2–FA–H2O solutions with the CV being highly rectangular in shape, suggesting
an insignificant effect of FA on the capacitance of CNTs. In the
CaCl2–FA solution, the CV currents were much smaller, which
could be due to the applied CaCl2 concentration (1 mol L 1)
was already close to saturation and the solution was very
viscous. Decreasing the temperature caused a significant
impact on the CVs recorded in the CaCl2–H2O and CaCl2–FA
solutions, but much less so in CaCl2–FA–H2O as exemplified in
Fig. 1b. Surprisingly, CVs were still recorded in CaCl2–H2O at
temperatures below the reported eutectic point, 51.6 1C.
These highly reproducible CVs did show noticeably increased
resistance in the cell (indicated by the sluggish current switching
at both ends of the potential scan), but the currents were not
insignificant. This observation indicates the presence of liquid
in the cell, likely due to supercooling of the CaCl2–H2O solution
(see ESI†).
The capacitance retention (CT/C20 = 100 capacitance at
designated temperature/capacitance at 20 1C) of the CNT
electrode as derived from the rectangular CVs (see ESI†) is
10820 | Chem. Commun., 2015, 51, 10819--10822
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Fig. 1 (a, b) CVs and (c) CT/C20 of a CNT electrode at (a) 20 1C and
(b) 60 1C in the CaCl2 solution of FA (1.0 mol L 1), H2O (2.0 mol L 1) and
mixed FA–H2O (1 : 1, v : v, 2.0 mol L 1). Potential scan rate: 100 mV s 1.
presented in Fig. 1c. At 20 1C, the measured specific capacitance, Csp,
was almost the same in CaCl2–H2O (108.3 F g 1) and CaCl2–FA–H2O
(107.6 F g 1). In this work, it was observed that the acid treated CNTs
exhibited 30–50% higher capacitance in the aqueous solution of
CaCl2 than that of KCl (see Fig. S4 in the ESI†). This difference
can be attributed to the greater affinity of the Ca2+ ions towards the
oxy-groups on the CNT surfaces as mentioned above. The fact that
each Ca2+ ion has two positive charges could be another important
cause for the increase of the specific capacitance. More discussion is
given later and also in the ESI.†
The dependence of specific capacitance on temperature was
studied next. In the temperature range from 20 to 70 1C, only
the CaCl2–FA–H2O solution could remain as a liquid, which is
needed to ensure ion mobility and accessibility to the electrode
surfaces. Consequently, a meaningful specific capacitance was
achieved (72.7 F g 1 or 67% capacitance retention at 60 1C,
see Fig. 1c). Fig. 2a shows the CVs of the CNT electrode in the
CaCl2–FA–H2O electrolyte at temperatures varying from 20 to
70 1C. In the experiment, the potential was cycled continuously,
and the CV was recorded at each of the indicated temperatures in
Fig. 2a. It can be seen that the CNT electrode exhibited highly
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Fig. 2 (a) CVs and (b) CT/C20 of a CNT electrode in the 2.0 mol L 1
solution of CaCl2 in mixed FA–H2O (1 : 1 v : v) at different temperatures as
indicated. Potential scan rate: 100 mV s 1.
satisfactory capacitive behaviour (rectangular shape) in the
applied temperature range. The CVs decreased in current as
temperature decreased, corresponding to the variation of specific
capacitance as shown in Fig. 2b. It can be noted that at 70 1C, the
CV is notably separated from that recorded at 60 1C, which can be
explained by the CaCl2–FA–H2O solution under supercooling (see
Fig. S1 and S2 in ESI†). Even though, the specific capacitance of the
CNT electrode was still measured to be as high as 53.4 F g 1 or 51%
capacitance retention. The cause for the capacitance decay with
decreasing temperature may be related to the reduced mobility
of ions at lower temperatures. A further contribution to lowering
the ion mobility may come from the organoaqueous solution
becoming more viscous upon cooling and supercooling.
Obviously, the dry ice container is not able to offer equilibrium
conditions at temperatures higher than that of dry ice. However, as
stated above, once placed in the dry ice container, particularly upon
the effect of opening and closing the container lid, the electrolyte
temperature dropped fairly slowly. At a rate smaller than 1 1C min 1,
the difference between CVs continuously recorded at 100 mV s 1 was
very much insignificant. Therefore, the results presented in Fig. 2
can be regarded as recorded under a pseudo-steady state, serving as
a practically useful guideline.
The higher specific capacitance of CNTs measured in aqueous CaCl2 electrolytes than in KCl warrants more discussion.
According to theory, electric double layer capacitance should
increase with electrolyte concentration.9a This trend agrees with
some previous studies of activated carbon in the 1 : 1 type electrolyte
such as KCl and KOH in which a double capacitance increase
required a 10 fold or greater increase of electrolyte concentration.9b,c
This journal is © The Royal Society of Chemistry 2015
Communication
To compare a 1 : 1 type electrolyte with a 1 : 2 type, e.g. KCl vs.
CaCl2, a simple way was adopted in this work to half the
concentration of the latter, e.g. 1.0 mol L 1 KCl vs. 0.5 mol L 1
CaCl2. The results obtained are presented in Fig. S4 in the ESI,†
showing clearly higher currents on the CVs recorded in the CaCl2
solution at both concentrations. Note that the CNT electrode
responded much more significantly to replacing KCl with CaCl2
than to doubling the electrolyte concentration. Thus, as discussed
above and further in the ESI† (Fig. S6), we believe that the unique
interactions between Ca2+ ions and the CNT surface oxy-groups
could have played important roles. This understanding is new
to supercapacitor research and, if further proven, could have a
significant impact on the development of supercapacitors
based on CNTs, graphenes and/or activated carbons which
are all enriched with various surface oxy-groups.
In summary, we have demonstrated that mixing CaCl2, FA
and H2O can enable not only very low freezing temperatures,
but also excellent capacitive charge storage in CNTs at both
room and sub-zero temperatures below 60 1C. The specific
capacitance of the CNTs measured in CaCl2 based electrolytes
is higher than that in more commonly used alkali salt based
electrolytes. This improvement can be largely attributed to the unique
affinity between calcium ions and the oxy-groups on the surfaces of
carbon nanotubes. We hope that this report can encourage further
research on using CaCl2 based electrolytes to improve the
performance of other carbon materials in supercapacitors.
This work has received financial support from the China
Scholarship Council, the EPSRC (EP/J000582/1), and the Ningbo
Municipal Government (3315 Plan and the IAMET Special Fund,
2014A35001-1).
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